A differential amplifier is a circuit that accepts inputs and determines a difference between them. Typical differential amplifiers take two inputs and generate a signal representing a voltage difference between the two inputs. Many circuits use difference comparators, such as analog to digital converters (ADC) or transceiver circuits.
Differential amplifiers are commonly used in communications systems to communicate signals between different circuits. For noise-immunity reasons, most communication standards utilize differential signaling. In differential signaling, data is transmitted in the form of a differential voltage across two nodes.
An ideal differential amplifier amplifies only the voltage difference between two inputs. Voltages common to both inputs are ideally rejected. However, due to differing communication standards, the common mode voltage utilized by various transmitters can vary widely between different transmitters. For example, some standards specify a high common mode voltage ranging between 700 mV and 1300 mV. Some standards, on the other hand, specify a low common mode voltage ranging between 145 mV and 350 mV, for example. In order to accommodate these various differential transmission circuits, a flexible receiver design needs to be able to accept different common mode voltages between the ground (0 mV) and the termination voltage (about 1200 mV).
Some previous approaches implement on-chip AC coupling capacitors on input signal lines to provide capacitive isolation between input terminals and a receiver circuit. The capacitive isolation allows the input common mode voltage of a receiver to be set independent of common mode voltage to the input terminals. However, if DC coupling is needed in a communication standard/protocol such as QuickPath Interconnect (QPI), where the receiver is directly connected to the transmitter without the AC coupling capacitors, the input common mode voltage may not be independently adjusted. Prior to reaching an analog front-end of a receiver, signals from a transmitter travel through backplane/cable that is generally lossy and behaves like a low pass filter, where high frequency components are attenuated and low frequency signals are largely unaffected. The transmission also results in intersymbol interference (ISI), where a current symbol will have an effect on an adjacent symbol when a sequence of random symbols is being transmitted. As a result, vertical eye opening of signals processed by the receiver are reduced and bit-error rate of the system is degraded.
One or more embodiments of the present invention may address one or more of the above issues.